1. Field of the Invention
This invention relates to a steam-drive process for producing viscous petroleum crude from a permeable earth formation and more particularly to a method for treating a subsurface permeable formation containing viscous petroleum crude during a steam-drive process to improve the sweep efficiency of the steam-drive process.
2. Prior Art
It has been known to steam-flood an earth formation containing viscous petroleum crude to increase the mobility of the crude and cause it to move to a producing location. In some steam-flood field procedures the same well is used for both steam injection and for crude production and other field procedures use separate injection and production wells spaced from each other through the petroleum containing formation. A variety of injection procedures using a variety of injection materials have been proposed most of which have the objective of increasing the efficiency of production of the petroleum crude at the producing well.
In the particular case of steam injection into and injection well with the objective of moving crude to a producing well, it has been observed that the efficiency of the sweep of crude from the formation is diminished, sometimes to zero, when injection steam breaks through into the producing well. This condition is known as gravity override. Hot water which separates from the injected steam tends to sweep through the bottom portions of the heated interval while the steam vapor tends to override the hot water, because of differences in densities of the two fluids. As the hot water flows through the reservoir, heat is transferred to the rock and reservoir fluids. This results in a temperature transition from the hot water bank to cooler water ahead of the bank. In practice, this means an ordinary waterflood precedes the warm and hot water banks which causes a gradual reduction in residual oil saturation with distance from the injector. Because of the unfavorable viscosity ratio the efficiency of this waterflood will be poor. However, good recovery efficiency with steam in the upper portion of the heated interval will result in significant reductions in residual oil saturations. These differences in oil saturations will adversely affect the naturally-poor relative permeability ratio of steam and water. As a result, injected steam will tend to prematurely breakthrough into the offset producing wells without sweeping the entire heated interval. If this condition is permitted to continue, the production of reservoir fluids can drop to zero and only steam and water will be produced at the producing wells.
In steam flooding, the rate of steam injection is initially high so as to minimize heat losses to the cap and base rock with time. Frequently, this procedure results in the development of a highly permeable and relatively oil-free channel between injector and producer. Many times this channel develops near the top of the oil bearing rock. In this case, much of the injected heat is conducted to the cap rock as a heat loss, rather than being conducted to oil bearing sand where the heat is needed. In addition, the steam cannot displace oil efficiency since little oil is left in the channel. Consequently, neither the gas drive from the steam vapor nor the convective heat transfer mechanisms work efficiently. This is why as a steam flood breaks through into the producing well it results in much lower oil recovery.
Further, while some graphic illustrations of steam profiles between injection wells and producing wells represent that steam and hot fluids start all along the injection well, rise toward the top of the producing interval in the direction of the producing well and drop down near the producing well, it is the present inventor's belief that such a profile is inaccurate. More than likely, once the steam has risen through the producing interval it will not drop down into the producing well when steam breakthrough occurs. This condition has been shown to exist by temperature profiles along a producing well. Such a condition further reduces the sweep efficiency of such a steam injection method.
It has been suggested to inject a blocking barrier into the formation above the steam injection to reduce the loss of steam through the breakthrough path. One such barrier is a foam as suggested in U.S. Pat. No. 3,412,793 issued to R. B. Needham on Nov. 26, 1968 for Plugging High Permeability Earth Strata. The highly permeable formation is temporarily plugged with a foam by introduction of steam and a foaming agent into the formation whereby a foam having steam as its gaseous phase is formed and, upon condensation of the steam due to loss of heat, the foam collapses. A similar procedure is shown in U.S. Pat. No. 4,086,964 issued to R. E. Dilgren et al on May 2, 1979 for Steam-Channel-Expanding Steam Foam Drive. That patent suggests the addition of a noncondensible gas to the foam and injection into the steam channel to provide foam and a relatively high pressure gradient within the channel. Neither of these patents are believed to provide the relatively permanent solution to the gravity override breakthrough problems as is disclosed in the present application.